In recent years, plastics have been used for optical materials, for example, lenses such as spectacle lenses, f.multidot..theta. lenses and pick-up lenses, prism, optical fibers and substrates of optical recording media or magneto-optical recording media. The plastics used for these optical materials are desired to have high transparency, good dimensional stability, high heat resistance and small birefringence. The optical materials are generally produced by injection molding, but the injection molding process includes high-speed flowing and high-speed cooling and thereby brings about orientation of resin molecules. As a result, molded articles having large birefringence are apt to be produced. Accordingly, there has been desired development of resins from which molded articles having not only excellent heat resistance and transparency but also small birefringence can be produced.